The manufacture of semiconductor devices and device carriers often requires the patterning of layers of materials that are deposited upon a substrate, such as a silicon wafer. In such processes a mask is generally used to form the pattern on the substrate. Since many devices comprise a number of different layers, it is necessary to precisely align the mask so that features on the various layers will have a predetermined relative position. In addition, it is important that alignment relative to the center of the substrate is precisely known so that optimal use of the surface of the substrate may be made. If the pattern is offset from the true center of the substrate features may be formed too close to the edge of the substrate or fixturing problems may arise in subsequent process steps.
It is often desirable to produce patterns upon both faces of a substrate; and, in such cases, a pattern on one face of the substrate often must correspond in some manner to the pattern on the opposite face of the substrate, as when elements disposed on the opposite faces need to be electrically or physically coupled. Careful alignment of the patterns on opposite faces of the substrate is necessary for the elements to be formed in the proper relative positions. For example, a signal line on one face of a substrate may be connected to a corresponding signal line on the opposite face of the substrate by a via that is formed through the substrate. In such a case, the two signal lines and the via must all be precisely aligned relative to one another. Hence, the precise alignment of patterns on opposite faces of a substrate is a required element of certain manufacturing processes.
A typical prior art method for obtaining precise alignment of patterns on both faces of a substrate uses sophisticated, relatively expensive, special-purpose machinery to mechanically align the patterning devices for the top and bottom faces of the substrate. Alternatively, Goirand, U.S. Pat. No. 4,937,162, discloses use of optically aligned mirrors to pattern the substrate coating. In either case, the machinery used is expensive and difficult to maintain, and therefore is not practical in many settings. In addition, the level of precision offered by such devices is unnecessary for certain applications which, nevertheless, require careful alignment of patterns. For example, in the manufacture of chip carriers, including multichip modules, the features are not as fine as those in a typical high density integrated circuit chip.
Other methods have been proposed using specific machinery with substrates having specialized physical properties, such as translucency as taught by Medernach in U.S. Pat. No. 4,371,598, or photoreactive doped materials as taught by Cade, U.S. Pat. No. 4,534,804. The use of such substrates permits construction of specialized devices to be used specifically with those substrates. Such methods are not general in nature, and, again, necessitate use of costly equipment which is often not justified by economic factors.
The present invention, therefore, is directed to a method for aligning patterns on opposite faces of a substrate which does not require expensive, specialized equipment for performing such an alignment. The method of the present invention can be performed using common equipment, without need for extensive reconfiguration of the equipment, particularly during use, as such reconfiguration would necessarily slow the manufacturing process. Extensive reconfiguration could lead to such delay and additional costs that would make it no more economical than use of specialized manufacturing machinery, as described above.
Accordingly, it is an object of the present invention to provide an improved method of aligning patterns on a substrate.
Another object of the present invention is to provide a method of aligning patterns on a substrate using common machining equipment with minimal reconfiguration of the equipment.
These and other objects of the present invention will become apparent to those skilled in the art from the following description and accompanying claims and drawings.